It had now been established that both the human and sheep fetus in utero make respiratory-like movements which occur intermittently and occupy about 30-40 percent of most hours of observation. The object of this project is to more completely examine the factors which control these fetal breathing movements. We propose to examine the contributions of peripheral (aortic and carotid body) chemoreceptors and of central (mid-brain) respiratory regulators. In the first year of this project we have compared the effects of respiratory (hypercarbic) acidosis and of metabolic acidosis on fetal breathing movements. Our studies are carried out in unanesthetized pregnant ewes with the fetus chronically catheterized for measurement of respiratory rate and depth.